Mass spectrometry and ion mobility spectrometry are analytical techniques for chemical analysis to detect and identify analytes of interest in various applications. With the increased use of these instruments, their applications and the variety of applications have increased. However, their size still remains large, hindering their applications in point of care/action/need applications, where size and portability is limiting.
A mass spectrometer is complex system composed of various components, as shown in FIG. 1. The critical components of a typical mass spectrometer consist of sample introduction and ionization 1, sampling inlet 2, ion optics and mass analyzer 4, detector 5, vacuum chamber or housing 3, vacuum system 9 including vacuum pumps and gauges, voltage supply systems 6, control systems 7, and data acquisition systems 8. In a typical mass spectrometer, first, the ionization source 1 ionizes a sample to generate positive and negative ions. The generated ions travel through the sampling inlet 2 and are guided, for example by ion guides, such as an ion funnel and/or multipole ion guides, to enter the mass analyzer 4. All of these components are closely connected to each other. The mass analyzer 4, which is derived by voltage supply systems 6, separates ions based on their m/z. The detector 5 produces an electrical signal when the ions hit the detector 5. The data acquisition systems 8 receive the electrical signal from the detector 5, typically in the form of electrical current or voltage, and produce and record spectra. The spectra provide fingerprints for chemical identification of the sample. Control systems 7 control various components. All components related to the mass analysis and ion detection are placed inside a vacuum chamber 3, maintained at high vacuum. Although FIG. 1A shows sample introduction/ionization block 1 outside the vacuum region, ionization of samples may occur in a wide range of pressures, from atmospheric pressure to high vacuum. In a conventional mass spectrometer, the sample introduction/ionization 1 is attached to the sampling inlet 2.
Mass spectrometers require high vacuum for proper operation because, ideally, ions must travel inside a mass spectrometer without colliding with background gas molecules. Therefore, the vacuum in the mass analyzer 4 of a mass spectrometer must be maintained at a pressure that correlates with ion mean free path length longer (ideally several folds) than the length of the mass analyzer. According to the kinetic theory of gases, the mean free path L (in m) is given by: L=kT/√2 pσ, where k is the Boltzmann constant, T is the temperature (in K), p is the pressure (in Pa), and σ is the collision cross-section (in m2). In a typical mass spectrometer with k=1.38×10−21 JK−1, T=300 K, and σ=45×10−20 m2, the mean free path equation simplifies to L=4.95/p, where L is in centimeters and p is in milli-Torr. In laboratory-scale mass spectrometers, ion filtering and detection usually occur in high vacuum, i.e. <10−5 Torr, corresponding to a mean free path of >4.95 meters. This is necessary to achieve high resolution separation of ions. To achieve a pressure of <10−5 Torr with available vacuum technologies, a two-stage vacuum generation process is utilized. First, the pressure is reduced to ˜10−2 Torr using mechanical or roughing pumps, and then one or more turbo-molecular pumps, ion pumps, or cryogenic pumps further reduce the pressure to <10−5 Torr. Turbo-molecular pumps provide relatively higher pumping capacities compared to ion pumps and are more appropriate for atmospheric pressure sampling and ionization. Ion pumps have advantages when vibration-free operation and ultra-high vacuum is required (vacuum levels of <10−10 Torr).
Prior to the introduction of soft ionization and ambient ionization techniques, mass spectrometry was generally limited to the analysis of volatile, relatively low-molecular-mass samples, and mass spectrometric analysis of biomolecules was difficult if not impossible. Also, conventional ionization sources, such as electron impact ionization, caused excessive fragmentation when applied to biomolecules. The advent of soft ionization techniques, which produce mass spectra with little or no fragmentation in ambient or near-ambient environment, made it possible to analyze large organic molecules and biomolecules with mass spectrometers. In particular, the development of electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) has extended the application of mass spectrometry to biomolecules. These techniques have demonstrated unparalleled advantages, for example in analyzing peptides and proteins, because of the speed of experiments, the amount of information generated, and the outstanding resolution and sensitivities offered.
Among various soft ionization techniques, ESI sources are best suited for direct biomolecules. ESI may function as a liquid sample introduction system and an ionization source at the same time. In ESI, the sample in a solution (typically a 50/50 mixture of water/methanol with 0.1-1% acetic or formic acid) enters a narrow capillary and leaves the capillary as a liquid spray. The voltage at the end of the capillary is significantly higher (3 to 5 kV) than that of the mass analyzer, so the sample is sprayed or dispersed into an aerosol of highly charged droplets. Evaporation of solvent decreases the size of the droplets. Because the electrically charged droplets retain their charge but get smaller, their electric field increases. At some point, mutual repulsion between like charges causes ions to leave the surface of the droplet. As a result, multiply charged ions from individual biomolecules, free from solvent, are released and enter the sampling inlet for analysis by the mass spectrometer.
Except for MALDI and similar ionization methods that ionize samples in the high-vacuum region, most mass spectrometry techniques for analyzing biomolecules rely on interfaces or sampling inlets that deliver gas-phase molecular ions from atmospheric pressure or near atmospheric pressure to high vacuum through orifices or capillaries. Achieving high ion transfer efficiencies for mass spectrometers is crucial and challenging. Conductance limiting orifice plates enable differential pumping of various stages of a mass spectrometer. Smaller orifices enable operation with lower pumping capacities but result in lower ion transfer efficiencies. Larger-diameter orifices may improve the efficiency of ion transfer but allow more neutrals to enter the vacuum region, thus requiring larger, higher-speed pumps to maintain the desired vacuum. Therefore, the pumping capacity of the vacuum system indirectly determines the ion transfer efficiency, because the size and dimensions of the sampling inlet must be designed according to the pumping capacity of the vacuum system. Finding the right balance between the pumping capacity and the ion transfer efficiency is a challenging design consideration for mass spectrometers if a limited pumping capacity is available.
Various sampling mechanisms are developed to address the above-noted challenges, such as the discontinuous atmospheric pressure interface (DAPI) and the pulsed pinhole atmospheric pressure interface (PP-API). The continuous atmospheric pressure interface enabled by differential pumping is another sampling mechanism that uses multi-stage vacuum pumps for differential pumping, to provide gradual pressure reduction to transport ions from atmospheric pressure to high vacuum. The extent to which the motion of ions may be controlled in different vacuum stages determines the overall ion transmission efficiency of the mass spectrometer. Recently, ion funnels have attracted significant interest in atmospheric pressure sampling in addition to the conventional multipole ion guides. Ion funnels enable the manipulation and focusing of ions in a pressure regime (0.01 to 30 Torr), providing much greater ion transmission efficiencies. Usually, ion funnels are located right after heated capillary inlets in a mass spectrometer. Ion funnels are rigid structures that guides ions in mid-vacuum level of 0.01 to 30 Torr. In ion funnels, the spacing between ring electrodes are constant.
Mass analyzers are the core components of mass spectrometers and are typically characterized by their mass range and resolution. Mass range is the maximum mass resolvable mass by the analyzer. Resolution is an indicator of how selective a mass filter is in distinguishing ions with m/z that are close in value. Thus far, various mass analyzers with different mechanisms have been developed. General mass spectrometry handbooks provide detailed descriptions of various mass analyzers. Mass analyzers may be categorized into beam analyzers, such as quadrupole and TOF analyzers, and trapping analyzers, such as ion traps.
Faraday cups and micro channel plate (MCP) detectors are the two most widely used ion detectors in mass spectrometry. Faraday cups may operate at high pressures (up to atmospheric pressure), but are less sensitive, and are not compatible with high-resolution mass spectrometry due to slow response times. MCPs support high mass resolution, dynamic range, and detection sensitivity. Most modern MCP detectors consist of two MCPs, with angled channels rotated 180° from each other, producing a chevron (v-like) shape. The angle between the channels reduces ion feedback. In a chevron MCP, the electrons that exit the first plate initiate the cascade in the next plate. The advantage of the chevron MCP over the straight channel MCP is significantly more gain at a given voltage. The two MCPs may either be pressed together or have a small gap between them to spread the charge across multiple channels.
With the advent of ambient desorption ionization sources, which desorb and ionize molecules in their native state, the applications of mass spectrometers have been extended significantly. For example, ambient desorption ionization techniques may be used to analyze human tissues during a surgery to differentiate cancer cells. As another example, ambient ionization desorption techniques may be used in homeland security to monitor cargo and passengers at security check points for explosives. Three different scenarios have been used thus far for such applications. In the conventional method shown in FIG. 1B, the samples are brought close to a mass spectrometer for ionization and analyses. In this approach, samples are directly place in front of a mass spectrometer. In a second approach shown in FIG. 1C, samples or sample molecules are transferred through a bare tube 19, which may be plastic or metal, into the ion source 11 of the mass spectrometer. A sampling medium, such as water, may be used to mix sample with sampling medium to be transferred through the bare tube to a mass spectrometer. In the third approach shown in FIG. 1D, samples are ionized using an ion source that is detached from a mass spectrometer and the produced ions are transferred via the bare tube 19 to a mass spectrometer for analysis. All of these approaches have disadvantages. For example, placing a sample directly in front of a mass spectrometer (FIG. 1B) may not be practical in many applications, particularly when the sample is bulky or immobile. Second transferring sample molecules via the bare tube 19 to a mass spectrometer (FIG. 2B) may result in memory effects from sample residue/molecules sticking to the inner surface of the bare tube 19. These residues may contaminate the inner side of the bare tube 19 and may adversely affect the analytical results. Transferring ions through bare tube 19, as shown in FIG. 1D, may result in decreased ion transfer efficiency as a majority of ions are lost to the inner walls of the bare tube 19 and deteriorate ion transfer efficiency. In other words, the ion transfer efficiency of this method may not be sufficient, and a majority of ions may be lost in the ion transfer process, thus negatively affecting analytical performance.